Coaxial NiO/Ni nanowire arrays for high performance pseudocapacitor applications

A new and low cost facile technique has been reported to fabricate coaxial nickel oxide/nickel (NiO/Ni) nanowire (NW) arrays as high performance electrodes for pseudocapacitors. The arrays are fabricated by direct electrochemical deposition of Ni inside nanoporous alumina template and the subsequent conversion of the outer shell into oxide by oxygen plasma annealing. This resulted in a unique core/shell, one-dimensional (1D) vertical nanostructure, which uses the inner Ni core as a conductor and robust support for a large effective surface area, and thereby provides reliable electrical connection to a thin redox-active NiO shell. The morphology, crystal structure, and shell thickness of NiO are found to be dependent on the annealing conditions (input power and plasma annealing time at a constant oxygen flow). The coaxial NiO/Ni NW arrays with the optimized redox-active NiO shell thickness displayed a high capacitance value of 0.36 F cm−2. Furthermore, the arrays show a rapid charge–discharge kinetics and stable long-term cycling performance (practically no drop in capacitance was observed even after 2000 cycles at high charging–discharging current of 5 mA cm−2). Excellent cycling stability and faster charge–discharge kinetics are attributed to the unique core/shell (NiO/Ni) and vertically aligned nanoarchitecture that facilitates faster electron and ion transport during the charge–discharge reactions.

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